How to tell if you are about to buy a juicy orange or grapefruit, no matter what the skin looks like:

• Pick up the orange or grapefruit. If it feels light, it's not juicy and will taste rather woody.
• Go through the bin picking the ones that feel heaviest compared to oranges or grapefruits of a similar size. They'll be the tastiest ones.

I was shown this trick by a very ancient and rather irritable gentleman when I was in my early 20's. He literally slapped the orange I was about to bag out of my hand and explained his system. Thanks to him, I've enjoyed decades of juicy oranges and grapefruits.